You can deploy new releases on one server of an existing multiplex without interrupting other servers, as long as the servers being upgraded are version 12.5 or higher. Always upgrade the write server last.
Any multiplex where all servers are not at the same version is mixed-version multiplex.
New features are completely available only after all databases are upgraded using ALTER DATABASE UPGRADE. If you upgrade databases on a query server, but leave the write server at 12.6, synchronization returns database versions on the query server to version 12.6, and you must repeat the ALTER DATABASE UPGRADE. Databases take their version from the write server.
Synchronizing affects database version, not the installed software version on a server. If you install 12.7 software on the query server, that software continues to be at version 12.7 after any synchronizations. Multiple database versions can exist on servers with installed software version 12.7, as shown in Table 1-4. However, you must connect to each server using an IQ agent of the same version, and the appropriate agent port.
Be sure to back up the write server before an upgrade. Once you create an IQ Local Store on a query server, you must back up that query server as part of future upgrades.
If you have a mixed-version multiplex with a 12.5 write server, be aware that creating a local store succeeds on a 12.6 or 12.7 query server but is not recognized by the 12.5 write server, and is lost at the next synchronization. (Version 12.5 did not support local stores.)
